1. Field of the Invention
The present invention relates to a radio frequency module that demultiplexes a transmission signal and a reception signal.
2. Description of the Related Art
A radio frequency module including a filter circuit has been previously provided. For example, a filter circuit 500 illustrated in FIG. 12 is mounted to a radio frequency module (see, e.g., Japanese Unexamined Patent Application Publication No. 2012-109818 (Paragraphs 0019 to 0023, FIG. 1, Abstract, etc.)). The filter circuit 500 includes a filter unit 503 connected between an input terminal 501 and an output terminal 502, and a path 504 connected in parallel to the filter unit 503. Thus, a signal path through which an RF (Radio Frequency) signal input to the input terminal 501 passes is branched into a path, which includes the filter unit 503, and the path 504. Therefore, when the RF signal is input to the input terminal 501, a first signal 505 passes through the filter unit 503, and a second signal 506 passes through the path 504. The first signal 505 having passed through the filter unit 503 and the second signal 506 having passed through the path 504 are combined, and a resultant RF signal is output from the output terminal 502.
The filter unit 503 includes a bandpass filter in which a predetermined pass band is set, and it allows the RF signal in the pass band to pass therethrough while attenuating the RF signal outside the pass band. However, in some cases, the RF signal outside the pass band cannot be attenuated to a desired value with only the filter unit 503. To cope with such a problem, a correction circuit defined by an inductor or a capacitor is disposed in the path 504. An impedance of the path 504 is set such that a phase of the second signal 506 passing through the path 504 and a phase of the first signal 505 passing through the filter unit 503 to be attenuated are opposite to each other in a frequency band outside the pass band of the filter unit 503, and such that an amplitude of the first signal 505 and an amplitude of the second signal 506 are the same in the relevant frequency band.
Accordingly, at a junction between a signal path on the output side of the filter unit 503 and a signal path on the output side of the path 504, the first signal 505 having passed through the filter unit 503 and the second signal 506 having passed through the path 504 cancel each other out in the frequency band outside the pass band of the filter unit 503, whereby the RF signal in the relevant frequency band and output from the output terminal 502 is attenuated. Thus, attenuation characteristics of the RF signal outside the pass band in the filter circuit 500 are improved. Furthermore, when another filter circuit defined by a bandpass filter for which a pass band different from that of the filter circuit 500 is set is arranged adjacent to the filter circuit 500, it is possible to suppress the RF signal outside the pass band of the filter circuit 500 from being output from the output terminal 502 and entering into the other filter circuit. Thus, isolation characteristics between the filter circuit 500 and the other filter circuit, which are arranged adjacent to each other, can be improved.
In the related-art filter circuit 500 described above, the path 504 including the correction circuit, which generates the RF signal having the phase opposite to that of the RF signal passing through the filter unit 503 and being outside the pass band, has to be disposed separately from the filter unit 503 in order to improve the attenuation characteristics for the RF signal outside the predetermined pass band. Accordingly, a problem arises in that the size of the filter circuit 500 increases and the size of a radio frequency module including the filter circuit 500 increases.